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Monday, January 28, 2013

Preparation of Anhydrous Nitric Acid - Lu Le Laboratory

Nitric acid (HNO3), also known as aqua
fortis and spirit of niter, is a highly corrosive strong mineral acid. The pure
compound is colorless, but older samples tend to acquire a yellow cast due to
the accumulation of oxides of nitrogen. Most commercially available nitric acid
has a concentration of 68%. When the solution contains more than 86% HNO3, it is referred to as fuming nitric acid. Depending on the
amount of nitrogen dioxide present, fuming nitric acid is further characterized
as white fuming nitric acid or red fuming nitric acid, at
concentrations above 95%. Nitric
acid is also commonly used as a strong oxidizing agent.

White fuming nitric acid, also called 100%
nitric acid or WFNA, is very close to anhydrous nitric acid. One specification
for white fuming nitric acid is that it has a maximum of 2% water and a maximum
of 0.5% dissolved NO2. Anhydrous nitric acid has a density of 1.513 g/mL and
has the approximate concentration of 24 molar. Anhydrous nitric acid is a
colorless mobile liquid with a density of 1.512 g/cm3, which solidifies at −42
°C to form white crystals. It boils at 83 °C. The anhydrous acid reqiures storage
below 0 °C to minimize decomposition. (Wikipedia)

White fuming nitric acid could not be
prepared from the distillation of 68% nitric acid, because water and nitric
acid form azeotrope at 121℃
. So if we want to get pure nitric acid, we could prepare it
from the distillation of the mixture of potassium nitrate and concentrated
sulfuric acid (98%).

When potassium nitrates react with sulfuric
acid, it produces nitric acid and potassium
bisulfate that is because the Ka1
value (dissociation constant) of sulfuric acid is about 105, the Ka value of nitric acid is 101.3 but the Ka2 value of sulfuric acid is
only 1.03x10-2. So if we
want to use the method to produce more anhydrous nitric acid, we should add
more sulfuric acid but not just dependent on the moles of protons.

KNO3 + H2SO4→ KHSO4
+ HNO3

In another view point, when we heat the
reacting mixture (KNO3 + H2SO4) , the boiling
point of pure nitric acid is 83℃ and sulfuric acid is 337℃ . So the reaction may tend to form nitric acid and potassium
sulfate at the boiling point of nitric acid (83℃).

KNO3 + KHSO4→ K2SO4
+ HNO3

To ensure our yield of anhydrous nitric
acid is higher, adding some excess sulfuric acid is very helpful. It makes the
final product less water and produces more nitric acid. So we add ~2 eq.
sulfuric acid into 1 eq. potassium nitrate to produce anhydrous nitric acid.

Chemical

1.Potassium nitrate: 30 g

2.Sulfuric acid: 98% 35mL

Procedure

1.Place 30 g potassium nitrate
into a round-bottomed flask.

2.Pour 35 mL 98% sulfuric acid
and place a stir bar into the flask.

3.Set up a simple distillation apparatus
and start heating the round-bottomed flask with a hot plate with stirring. Soak
the collecting vessel in a cold-water bath.

Hi Lu Le ! I must admit it is so refreshing to find someone who is so open and easy going, I'm sure you know what I mean, the more qualified organic chemist's become, I feel they loose their "casual fun" nature for enjoying even the simple things in chemistry! (are you thinking-this isnt organic chemistry! oh, but it is! because concentrated nitric acid always loves to wreck carbon based molecules! like the way it is attacking the vacuum tube on your condenser! ha! (joke) anyhow, without having to research too much, I wonder if I may just ask you this-what would be the most single most effective way of keeping the yellow NO2 out of end product? more precise temp. control? higher vacuum? I know for nearly all purposes it doesnt matter, but just for asthetics, you know, its nice to have something "pure" , yes?.

Hi psycronizer~ Ha ha~ You got the point. Many people asked me the same question about the removal of excess NO2 from the nitric acid. A recommended way to remove NO2 from the fresh distilled nitric acid is bubbling dried nitrogen gas or CO2 gas to drive out the solving NO2. This method is an application of Dalton's partial pressure rule. Another way, which I just figured out but never tried, is adding some "H2O" to react with NO2 and then producing HNO3 until the yellow or brown color is vanished. This method is worth to try, but the controlling of addition of water could be very important. Adding water drop by drop may be the simplest way. The other way, I think it may work well, is as follow: 1. Prepare a sealable container large enough to put other two smaller beakers in. 2. Pour the fresh distilled HNO3 in one beaker, and fill another beaker with some water. 3. Put these two beakers in the same container and seal it. 4. Just wait for hours for reaching the pseudo-equilibrium of vapor pressure and the yellowish color, NO2, in the HNO3 may be quenched by water vapor which is from another beaker. (These are all by my prediction :D, if you found it works well please inform me and also put my name in your article as a coauthor~ :D)